The collection of only plasma from volunteer donors, as opposed to the collection of whole blood, is not widespread. As a result, much of the plasma now collected for fractionation purposes comes from paid donors, not volunteer donors. This circumstance can be attributed to several, interrelated factors. First, conventional disposable whole blood collection systems are not suited for the collection of relatively large pools of plasma from which the various therapeutic plasma proteins, such as albumin and AHF (anti-hemophilic factor), are obtained. On the other hand, conventional disposable plasma collection systems involve a process (called plasmapheresis) which is time-consuming and, in part for this reason, does not appeal to volunteer donors. Furthermore, during the plasmapheresis process, while the whole blood is being separated into red blood cells and plasma, the blood collection system (typically a series of integrally attached bags) is physically separated from the donor. Such physical separation requires procedures to minimize the risk of error when several donors are being processed simultaneously that one donor's red blood cells are not inadvertently returned to another donor. In addition, physical separation of the blood from the donor could potentially raise concerns in the collection staff of exposure to infectious agents in the collected blood if fluid drips or leaks occur.
Second, while on-line extracorporeal separation systems, in which the blood collection system is not physically separated from the donor during the collection procedure, are also known, such systems are generally expensive, complex, not "donor-friendly", and are generally unsuited for portable operation.
For example, a representative centrifuge-based system is disclosed in Judson et al. U.S. Pat. No. 3,655,123 entitled "Continuous Flow Blood Separator."
A representative membrane-based system is disclosed in Popovich et al., U.S. Pat. No. 4,191,182.
One system of membrane collection suitable for portable operation has been described in a published European Patent Application, Publication No. 0114698 published Aug. 1, 1984; entitled "Process and Apparatus for Obtaining Blood Plasma." In this system, a unit of blood is withdrawn from a donor into a set containing a membrane filter, tubing and a sterile blood container (such as a conventional blood bag). The whole blood is first passed through the filter. The plasma flows through the membrane filter and is collected in a separate plasma container. The remainder of the blood unit, which had passed from the inlet to the outlet of the filter, is accumulated in the sterile container. It can then be immediately returned to the donor.
In this approach, the pressure available for driving the filtration process and for propelling the blood from the inlet to the outlet of the filter is relatively small. This pressure includes the donor's venous pressure (which, with an inflated pressure cuff on the donor's arm, is on the order of 40 mm Hg) and available hydrostatic head (approximately 50 mm Hg) for a total pressure on the order of 90 mm Hg. These pressures may vary significantly from donor to donor. This can result in a relatively slow and variable plasma collection time. It also requires relatively large filters to function at the available low driving pressures. It can also be difficult to achieve precise anticoagulant flow proportional to blood flow with inexpensive and simple-to-use hardware.
Another membrane-based system is disclosed in a group of three U.S. Pat. Nos. 4,479,760 entitled "Actuator Apparatus for a Prepackaged Fluid Processing Module Having Pump and Valve Elements Operable in Response to Applied Pressures"; 4,479,761 entitled "Actuator Apparatus for a Prepackaged Fluid Processing Module Having Pump and Valve Elements Operable in Response to Externally Applied Pressures"; and 4,479,762 entitled "Prepackaged Fluid Processing Module Having Pump and Valve Elements Operable in Response to Applied Pressures," all issued to Bilstad et al. The system of the Bilstad et al. patents utilizes a disposable module containing a hollow membrane filter, a plasma container and other elements. A fixture is provided to receive the module during the donation cycle. Constant volume pumps in the fixture are provided to draw whole blood from the donor into the inlet side of the filter and to return the concentrated red cells to the donor from the outlet side of the filter. A single needle is used from both drawing the whole blood from the donor and returning concentrated red blood cells to the donor.
Various other types of membrane filters for plasmapheresis are also known. For example, U.S. Pat. No. 4,212,742 to Solomon et al. discloses a filtration device that utilizes planar membrane elements suitable for use with transmembrane pressures in a range of 180 mm Hg down to 100 mm Hg. U.S. Pat. No. 4,381,775 to Nose' et al. discloses a cylindrical filtration device that utilizes hollow membrane filters. The filter disclosed by Nose' et al. was intended to operate with transmembrane pressures in a range of about 50 mm Hg down to 8.5 mm Hg.
A commercially available, cylindrical membrane plasma filter in a set sold by Baxter Travenol Laboratories, Inc., the CPS-10. This filter is, relatively speaking, rather large. This filter has a fiber length in the order of 20 cm resulting in an overall length of about 29 cm. With an input blood flow rate of 70 ml/minute and a plasma output flow rate of 20 to 25 mil/minute, the internal diameter of each of the fiber members is about 320 microns, with 800 fibers needed. The CPS-10 filter is normally operated with an inlet port to outlet port pressure drop on the order of 70 mm Hg.
It would be desirable to make the collection of plasma a volunteer-based activity to a much greater extent than it is currently. The ability to provide filters having optimal performance characteristics, given the constraints of a "donor-friendly" blood collection system, would be a step forward toward this worthwhile objective.